1) formats
Every reader reads only one drm format, but multiple non-drm ones.

Nope - not true. The only restriction (and it's a contractual thing) is that no device can support DRM Mobi and another DRM format. There are no problems with other combinations. Eg, Sony devices read DRM ePub, PDF, and Sony's own "LRX" DRM format. The B&N nook reads both eReader and ePub/PDF DRM.